1. Field of the Invention
This invention relates to an electrophotographic method using a screen-like photosensitive medium having a number of fine openings (hereinafter referred to as the screen), and more particularly to such a method which enables good formation of images.
2. Description of the Prior Art
In a conventional electrophotographic method, an electrostatic latent image is formed by a predetermined process on a photosensitive medium in the form of a plate comprising superposed layers, whereafter the latent image is developed and the developed image is transferred onto paper or other transfer medium. On the other hand, the photosensitive medium is cleaned after the image transfer by cleaning means such as a brush or a blade member, to remove any unnecessary developer therefrom in preparation for another cycle of image formation. Such conventional cleaning means intensely rubs the surface of the photosensitive medium, thus resulting in damage to and reduced durability of such surface.
Also, the conventional electrophotography which involves the step of cleaning has accordingly required a space available for the cleaning step, to be provided adjacent to the photosensitive medium, and the cleaning means which has a complicated mechanism in itself has led to complication of the entire image formation apparatus. To overcome the problems concerning the durability of the conventional photosensitive medium and the complication of the apparatus, there is a method known as the transfer of electrostatic image (TESI). In this TESI method, the latent image on the photosensitive medium is transferred onto a transfer medium, and then the transfer medium is developed to form a visible image. Thus, the photosensitive medium is not subjected to development and this eliminates the need to provide the cleaning means for the photosensitive medium, and accordingly eliminates the possibility of reduced durability of the photosensitive medium which would otherwise result from the friction imparted thereto and this has raised the expectation of increased service life of the photosensitive medium. However, the TESI method is disadvantageous in that the final image formed thereby is faster to reduce its quality than the final image formed by the conventional electrophotographic method using the cleaning means. Such an unexpected phenomenon may be considered attributable to the corona discharge imparted to the photosensitive medium during the formation of the latent image. More particularly, substances produced by the corona discharge and dust or like materials in the air are deposited on the photosensitive medium and since these substances and dust or like materials are moisture-absorbent or electrically conductive, they seem to reduce the electrical resistance on the surface of the photosensitive medium and prevent the electrostatic latent image from being formed with a sufficiently high potential. Other various electrophotographic methods are known which use a screen but which do not need the cleaning means for the photosensitive medium, as in the core of above-described TESI method. These electrophotographic methods are disclosed, for example, in U.S. Pat. No. 3,680,954, U.S. Pat. No. 3,582,206, U.S. Pat. No. 3,645,614 and U.S. application Ser. No. 480,280, filed June 17, 1974, now abandoned, wherein the primary electrostatic latent image formed on the screen is not developed but ion flows are modulated by the latent image to thereby form a secondary electrostatic latent image on a recording medium. These electrophotographic methods whereby the image formation is achieved by ion flows being modulated by the use of a screen do not need the use of the cleaning means and therefore, a phenomenon similar to that peculiar to the TESI method occurs in an image formation apparatus having no cleaning means.
To prevent any reduction in electrical resistance of the surface of the photosensitive medium occurring in the above-described TESI method, there is known a method of applying conventional cleaning means to the photosensitive medium or a method of supplying silicone oil to the surface of the photosensitive medium and further cleaning such surface likewise. According to these methods, the reduction in electrical resistance of the photosensitive medium may be prevented to some extent, but the reduction in durability of the photosensitive medium resulting from the friction imparted thereto by the cleaning means is unavoidable. Of course, application of cleaning means to the electrophotographic method using a screen would also be conceivable, whereas direct application of conventional cleaning means would be difficult. More specifically, the substrate of the screen is formed by a net of metallic fine wire or by a relatively deformable blank shaped by electrotyping a sheet of metal and this offers a problem of whether or not the screen itself can withstand the friction imparted thereto by the cleaning means or whether or not the openings of the screen can be cleaned as well.